As it is well known in the art, in smart power devices a linear control circuit is generally capable of driving a power element and allowing the highest voltage and deliverable current to be provided and diagnostic functions to be performed. These smart power devices are powered in two different modes. In a first mode, the control circuit receives a trigger signal and is powered by a battery. In a second mode, the control signal receives a trigger signal, and is also powered by the trigger signal, and is not coupled to the battery.
In particular, FIG. 1(a) shows a known solution. A control block 1 drives a first power element, in particular an IGBT transistor TR1 of electronic device 2. Control block 1 is powered by a supply voltage Vbat and activated by a trigger signal VTRIGGER.
The first power element TR1 is inserted between a first voltage reference, in particular a supply voltage Vbat and a second voltage reference, for example a ground reference GND. The first power element TR1 has a first conduction terminal, in particular a collector terminal C, coupled to the first voltage reference Vbat, a second conduction terminal, in particular an emitter terminal E, coupled to GND, as well as a control terminal, in particular a gate terminal G coupled to control block 1. The first power element TR1 is also coupled to an inductive load, for example a primary winding 3A of a coil, coupled in turn to the supply voltage reference Vbat and to an igniter plug 3B of the coil itself.
A second power element, in particular an IGBT sensing transistor TR1SENSE, and a sensing resistance Rs are coupled in series to each other and in parallel between the terminals C and E of the first power element TR1.
During conduction, the two power elements TR1 and TR1SENSE sink different fractions of the same conduction current ICOIL.
FIG. 1(b) shows a second known solution. FIG. 1(b) shows a control block 10, which is changed with respect to the control block 1 shown in FIG. 1(a). The supply voltage of block 10 is provided by the voltage at the high state of the trigger signal, as well as being used to activate the control block 10.
For the smart power devices such as those shown in FIGS. 1(a) and 1(b), the lowest and the highest output current ICOIL and the lowest and the highest voltage of control signal VTRIGGER are specified. It is necessary that the smart power device 2 shown in FIGS. 1(a) and 1(b) correctly operates even in the worst case operating situations. A worst case operating situation occurs for low battery voltages Vbat at extreme temperatures, when the trigger voltage VTRIGGER, at the high state, can be reduced. For example, the signal ground can be separated from the power ground, and the real voltage being applied between the gate G and emitter E terminals of the first power element TR1 is further reduced by the voltage drop ΔV introduced by connection cables and connectors.
FIG. 2 shows the same control block 10 of FIG. 1(b), wherein the voltage drop ΔV introduced by cables and connectors is shown.
For electronic ignition applications, in the automotive field, under “normal” operating conditions, it is traditionally required that a power element, in the case of FIG. 2 the first power element TR1, is capable of delivering an output current ICOIL no lower than 17 amperes, with a trigger signal VTRIGGER at the input of the block whose level is equal to 5 volts. Under worst case operating conditions, however, the voltage value of the trigger signal VTRIGGER can be reduced down to 2.5 volts.
In the non-limiting case wherein the power element TR1 is an IGBT transistor, the highest voltage applied on the gate terminal G is given by the trigger voltage VTRIGGER minus the voltage drop ΔV introduced by the control block 10. This voltage also determines the highest output current ICOIL deliverable by smart power device 2.
In this situation, it is very difficult to meet the required specification concerning the minimum output current ICOIL of 17 amperes, with the reduced-voltage trigger signal VTRIGGER, unless an IGBT transistor with an oversized area is used.
FIGS. 3(a) and 3(b) shows, in a series of graphs, signals related to simulations on the circuit of FIG. 2 performed for the first power element TR1. In particular an IGBT transistor whose active area is equal to 10 mm2, driven by the control block 10 limiting the output current ICOIL to 20 amperes was used.
FIG. 3(a) relates to the case of the trigger voltage VTRIGGER at the high state of 2.5 volts and FIG. 3(b) shows the simulation results as the amplitude of the voltage varies.
When the trigger voltage VTRIGGER is 2.5 volts [V(TRIGG—1)], the output current I(COIL—1) stays lower than 4 amperes, which is quite lower than the predetermined limitation current, since the actual voltage V(GATE—1) calculated on the gate terminal G corresponds to about 2.3 volts.
When the trigger signal VTRIGGER reaches a low state, the IGBT transistor TR1 is disabled and the power accumulated in the primary winding 3A of a coil transfers to the secondary winding generating a spark on the igniter plug 3B.
When the current in the collector terminal C of the IGBT transistor TR1 is too low, the accumulated power can be insufficient for generating the mixture combustion in the explosion chamber, with the subsequent misfiring phenomenon which is, as is well known, very damaging for the motor.
From the simulations of FIG. 3(b) it can thus be appreciated how the current ICOIL, to reach the limitation value of 20 amperes, requires a trigger signal VTRIGGER whose amplitude is no lower than 4 volts.
To overcome the above-mentioned problems, the prior art suggests the use of a charge pump fed by a trigger signal VTRIGGER, capable of generating an output voltage being sufficiently high as to conveniently bias the gate terminal G of the IGBT transistor TR1, allowing thus the current ICOIL required by the application to be delivered therefrom.
Although this solves the problem, the solution has a serious drawback. The noise generated by the inner oscillator of the charge pump can cause electromagnetic noise making the device incompatible with emission regulations.
Moreover, this noise, which is also reflected on the voltage of collector terminal C, is transferred to the coil secondary winding, by means of the turn ratio, which can generate undesired overvoltages.
To address these problems, it would be thus necessary to further increase the circuit complexity of the control block 10 by inserting filters.
What is desired, therefore, is a control circuit for a smart power device having a reduced amplitude trigger signal, such that a sufficiently high current is produced to avoid ignition problems, but overcomes the problems associated with the prior art solutions described above.